In recent years, optical information recording media have been required to have larger and larger information recording capacity for processing of a vast amount of information such as images. As a method of solution of this, there is a method using the super-resolution technology which is one of the information processing enhancement technology at the time of reproduction.
The super-resolution technology is the technology for reproducing a signal of a mark length which is shorter than an optical system resolution limit (a limit determined by a laser wavelength and the numerical aperture of an optical system) of a reproduction apparatus. This makes it possible to perform recording using a smaller mark length, whereby substantial recording density is increased. This is because what becomes a problem in achieving higher density is a reproduction technology, not a recording technology.
Of these technologies, the super-resolution technology will be described first.
In the past, many optical information recording media (hereinafter referred to as super-resolution optical information recording media or super-resolution media) for reproducing a signal of a mark length which is shorter than an optical system resolution limit of a reproduction apparatus have been proposed.
As such a technology, as a technology that can be applied to a read-only medium on which non-rewritable information is recorded as depressions and projections of a substrate, there is the technology for providing a layer called a functional layer formed of a thin metal film or the like on a substrate on which information is recorded as any one of depressions and projections or both (refer to PTL 1).
Although most of the principles of the above-described super-resolution medium related to PTL 1 have not been made clear at present, a signal of a mark length which is shorter than an optical system resolution limit can be reproduced by a change in the temperature of the above-described functional layer.
Moreover, as another technology to which a read-only medium can also be applied, the technology for providing, as a mask layer, a thermochromic pigment layer whose optical performance (transmittance) varies with a temperature on a reproduction light incidence surface of a reflective film has also been known (refer to PTL 2).
Incidentally, the mask layer is a layer that causes a super-resolution phenomenon by, for example, reducing a laser spot, which will be described later, in a pseudo manner.
In these optical information recording media, the fact that a light intensity distribution is present in a laser spot caused by a reproduction laser with which a reproduction surface is irradiated and a temperature distribution is caused thereby is used.
More specifically, in a reproduction laser spot on a mask layer which is closer to a reproduction light incidence surface than a reflecting layer, a temperature or light intensity distribution is caused, which causes the distribution of optical performance in the laser spot.
For example, if a material whose transmittance becomes higher when the temperature rises is used as a mask layer, since only the transmittance in a high-temperature portion becomes higher, a laser spot that is generated on a reflecting layer surface is reduced in a pseudo manner. As a result, it is possible to reproduce a signal of a mark length which is shorter than an optical system resolution limit.
However, in the super-resolution reproduction technology, since a laser spot is reduced in a pseudo manner by masking a laser light, the use efficiency of a reproduction light is reduced (a reflected light from a reflecting layer is naturally reduced). As a result, a limitation is imposed on a reduction of a laser spot, and enhancement of recording density has been limited to about double density in terms of linear density.
In the past, as described above, a method for increasing the capacity of the optical information recording medium using the super-resolution technology has been proposed.